The present invention relates to an FM intermediate-frequency circuit which at least includes a limiter-amplifier, a level detector, and a phase-locked loop containing a VCO oscillator. The level detector is incorporated into the limiter-amplifier to which the intermediate-frequency signal is applied.
One conventional FM intermediate-frequency circuit is shown in FIG. 1. In this figure an intermediate-frequency signal IF is fed to the limiter-amplifier (1) which contains a level detector 5 having an output signal I. The phase detector (2), the loop filter (3), and the VCO (4) form a phase-locked loop with an output FM demodulation (FM-dem) signal.
Assembling the elements of an intermediate-frequency circuit such as this, in which the FM detector is a PLL loop (which is made up of a phase detector, a loop filter and a VCO), results in problems for the total amplification of the limiter-amplifier. Excessive amplification will produce detrimental oscillation in the circuit, i.e. the limiter-amplifier will amplify only the VCO signal, the detection of the incoming signal being prevented. Therefore, the maximum usable amplification of the limiter-amplifier is approximately 50-60 dB.
When internal level detector of the limiter-amplifier is activated, its maximum dynamic range detection capacity is equal to the total amplification of all of the limiter-amplifiers within the circuit, i.e. according to the discussion above, 50-60 dB. This dynamic range is, however, too low for radio telephone applications, for example, since these applications typically require a dynamic range of approximately 75 dB.
There are on the market very few intermediate-frequency circuits in which the FM demodulation of the signal has been implemented by means of a PLL, and usually these circuits do not have level detection of the input signal. If the total amplification of the limiter-amplifier is, for example, over 90 dB, the PLL detector can be implemented in such a way that the VCO is in another circuit at a sufficient distance from the input of the limiter-amplifier. The condition is thus that the isolation requirement for avoiding oscillation is fulfilled.
There is no precise numerical value for the amplification of the limiter-amplifier. What is most important is that the total amplification of the system sufficient to bring the output of the limiter-amplifier circuit up to the limit. If the amplification of the limiter-amplifier is "deficient", i.e. it is not limited to its own noise, the missing amplification must be made up by amplification of the pre-stages, and vice versa.